With the progress in compact design and performance improvement in the field of electronic equipment production, an increasingly high density design has come to be required in the design of printed circuit boards used in the electronic equipment as exemplified in the increase in the number of layers of the printed circuit board, the reduction in the thickness of the board, and the reduction in the diameters and the spacing of the through holes. According to a more recent proposal, semiconductor chips are directly mounted on a printed circuit board, and the entire assembly is sealed in a resin package. Semiconductor packages such as plastic pin grid arrays and plastic ball grid arrays often consist of printed circuit boards. A printed circuit, when applied to a semiconductor package, may be subjected to temperatures exceeding 175.degree. C. for wire bonding and resin sealing during the manufacturing process. In such a case, if the mechanical strength or the resiliency of the printed circuit board is insufficient, it could lead to various problems such as poor connection of the bonding wire, and warping and twisting of the circuit board following the sealing process. To achieve favorable material properties such as hardness and resiliency at high temperatures in excess 175.degree. C., Tg (the glass transition temperature) must be raised to a level which has not been hitherto possible. Also, because the printed circuit board for a semiconductor package requires an extremely high density wiring pattern, the reliability in ensuring electric insulation is important for the material of the printed circuit board.
To meet such requirements, there have been proposals to raise Tg (the glass transition temperature) of epoxy resin for printed circuit boards. For instance, it was proposed to cure multi-functional epoxy resin by using dicyandiamide in Japanese patent laid open publication (kokai) No. 60-155453). However, the epoxy resin which is cured by using dicyandiamide tends to absorb moisture, and is therefore known to be inadequate for ensuring the level of electric insulation capability which can meet the demand for insulating printed circuit boards of high density design. In particular, migration of the material of the metal, which forms wiring, circuit patterns and terminals in or on the printed circuit board, on or inside the insulating material under the influence of high temperatures and voltage differences poses a serious problem.
On the other hand, the printed circuit board obtained by curing epoxy resin by using multi-functional phenol resin has a significantly loss tendency to absorb moisture, and therefore demonstrates a better ability to avoid metal migration. However, a printed circuit board made by using multi-functional phenol may fade in color during the heating process depending on the kind of the phenol used. In Japanese patent publication (kokoku) No. 62-28168, it was proposed to use material mainly consisting of phenol or bisphenol A mixed with high orthophenol-formaldehyde resin to avoid the fading of the printed circuit board, but Tg which allows the printed circuit board to withstand temperatures in excess of 175.degree. C. cannot be attained.
Also, when multi-functional epoxy resin is cured by multi-functional phenol, Tg may be raised to a sufficient level, but the cured resin becomes so rigid and inflexible that the adhesion to the metallic foil may become poor, and small cracks may be produced when through holes are drilled in the printed circuit board. Such small cracks are known to cause metal migration, and are highly detrimental to the reliable insulation of the printed circuit board.